The present invention relates to the field of wing structures, and in particular to aircraft wing structures. The present invention also relates to a method of assembly of a wing structure and to a wing spar.
An aircraft wing box is a wing structure and generally includes the structural members of the wing such as front and rear spars, ribs, braces, struts etc. The upper and lower skins of the wing enclose the wing box. Generally, the roots of the front and the rear spars are connected to the fuselage.
The spars and other structural members of a wing structure are generally fastened together using metal fasteners, such as rivets or metal nuts and bolts. These metal fasteners extend through the thickness of the members to be fastened. For example, a spar is usually fastened to a wing skin by fastening a metal fastener through a hole in a spar cap of the spar and a corresponding hole in the wing skin. Hence, the metal fastener extends from the outside surface of the wing to an underside of the spar cap.
Most large aircrafts have fuel tanks inside the wings. The fuel tanks inside the wings are generally contained between the front and rear spars. Therefore, the metal fasteners used to fasten the spars to the wing skins extend from the outside wing surface into the fuel tank.
Since aluminium alloy, commonly used in aircraft, is highly electrically conductive, a transient charge from lightning striking a metal fastener discharges into the rest of the aircraft structure with current from the discharge being distributed relatively evenly over the aircraft. Therefore, a typical lightning strike to a metal aircraft causes either no or only minor damage to aircraft components.
Composite materials (such as carbon fibre) are increasingly being used in aircraft manufacture due to their higher strength-to-weight ratio compared to aluminium alloy. However, typical composite materials are much more electrically resistive than aluminium alloy. For example, Carbon Fibre Reinforced Plastic (CFRP) is at least 2000 times more resistive than aluminium.
As the composite material is an electrical insulator, any lightning that hits the metal fastener is not easily dissipated within a composite wing skin. This can increase the risk of sparks or ignition of fuel in the region of the fastener. Furthermore, metal fasteners on an exposed composite surface are most susceptible to a direct lightning strike.
Accordingly, composite structures on an aircraft typically have some protection where a metal fastener extends into a fuel tank region. This is because any ignition events in a region of the metal fastener could risk ignition of fuel in the fuel tank. Unfortunately, typical lightning strike protection approaches are complicated and difficult to implement. For example, complex sealing or electrical bonding techniques are often used.
The present invention seeks to provide a wing structure, especially a composite wing structure that provides improved lightning protection. Additionally, or alternatively, the invention seeks to provide a wing structure, especially a composite wing structure that provides improved bird strike protection. Additionally, or alternatively, the invention seeks to provide an improved wing structure.